Power returnable coilable rules are widely employed because of the convenience and ease of operation which they afford the user. To hold the blade in an extended position against the retracting force of the power spring, numerous types of locking devices have been proposed and utilized. A highly effective lock is illustrated and claimed in U.S. Pat. No. 3,214,836, granted Nov. 2, 1965 to the assignee of the present invention as the assignee of Robert F. West.
As the length of the measuring blade increases, the available spring power exerted by the recoil spring must be increased. Accordingly, the gripping or clamping force of the blade lock must be strong enough to withstand not only the spring force but also the pull of the user as he or she pulls the blade taut between the hook at its outer end and the casing. However, the lock must still permit facile movement of the blade inwardly and outwardly of the casing as well as permit rapid locking and unlocking.
It is an object of the present invention to provide a power returnable coilable rule employing a novel blade locking mechanism which will securely grip the blade to hold it in an extended position and permit tensioning of the extended portion by the user, and which is readily releasable.
It is also an object to provide such a coilable rule in which the blade locking mechanism is comprised of relatively durable and easily fabricatable parts to provide a long lived assembly.
Another object is to provide such a coilable rule which is relatively economical and in which the parts may be readily assembled.
A further object is to provide such rule in which the lock button is movable among any one of three positions--a normal blade locking position, a finger-pressure blade releasing position, and a releasably engaged position in which the blade is freely movable.